Wireless network topologies that enable wireless nodes (i.e., mobile stations, wireless terminals) to communicate with each other and with fixed networks generally fall into two categories: i) infrastructure-based and ii) infrastructureless. Infrastructure-based networks have traditionally been based on the cellular concept and require a high level of infrastructure support. In an infrastructure-based network, wireless nodes communicate through access points (e.g., base stations) connected to the fixed network (e.g., Internet). Typical infrastructure-based networks include GSM networks, UMTS networks, CDMA networks, WLL networks, WLAN, and the like.
In an infrastructureless network, wireless nodes (i.e., mobile stations, wireless terminals) communicate directly rather than through access points or other base stations. One common and increasingly popular infrastructureless network topology is the mobile ad hoc network (MANET). A MANET is a group of wireless nodes that dynamically form a network with each other and without using any pre-existing fixed network infrastructure. In most cases, wireless nodes of a MANET are small mobile devices that are relatively limited in term of CPU capability, memory size, and power consumption.
Mobile ad hoc networks are expected to continue to grow over the next 2-3 years to become the dominant peer-to-peer communication technology. Cell phones are being equipped with IEEE-802.11 and other wireless LAN technologies. The proliferation of cell phones and the ubiquity of economical IEEE-802.11 networks will create a new kind of mobile, ad-hoc and peer-to-peer network.
As noted above, a mobile ad hoc network (MANET) can be set up without using a pre-existing network infrastructure. This can be done anywhere and at anytime. The wireless nodes of a MANET are connected by wireless links and are free to move randomly. The wireless nodes also act as routers. A MANET supports traffic types that are different from the traffic types typically found in an infrastructure-based wireless network. MANET traffic types include: 1) peer-to-peer traffic; 2) remote-to-remote traffic; and 3) dynamic traffic.
In peer-to-peer traffic, there is only one hop between the communicating wireless nodes (i.e., direct communication). In this instance, the network traffic (in bits/second) is usually constant. In remote-to-remote traffic, there are two or more hops between communicating wireless nodes, but a stable route is maintained between the source and destination nodes. This often occurs if several nodes stay within range of each other in one area or if the nodes move as a group. Dynamic traffic results when the MANET nodes move around and communication routes must be reconstructed. This often causes poor connectivity and network traffic occurs in short bursts.
Each MANET node is autonomous and may function as both a host and a router. Thus, each wireless node performs basic host processing and performs router switch functions. Thus, endpoints and switches are indistinguishable in a MANET. Since there is no central network to control network operations, control and management of a MANET is distributed among the wireless nodes. The MANET nodes cooperate to implement security and routing functions.
A MANET may implement different types of routing. Basic types of ad hoc routing algorithms are single-hop and multi-hop. These are based on different link layer attributes and routing protocols. A single-hop MANET is simpler than a multi-hop MANET, but lacks the functionality and flexibility of a multi-hip MANET. When delivering data packets from a source to its destination out of the direct wireless transmission range, the packets should be forwarded via one or more intermediate nodes.
Since MANET nodes are mobile, the radio frequency (RF) links may change rapidly and unpredictably over time. In order to compensate for traffic and propagation conditions, the MANET nodes dynamically modify routing information between each other as the nodes move, thereby forming new network topologies. Advantageously, a MANET node may operate not only within the mobile ad hoc network, but also may access a public fixed network (e.g., cellular network).
MANET nodes may use, for example, an Ad Hoc On-Demand Distance Vector (AODV) routing protocol in ad hoc network environments in which the movement of the MANET nodes cause frequent changes in RF link quality. The AODV protocol enables the MANET nodes to adapt quickly to dynamic link conditions. The AODV algorithm enables dynamic, self-starting, multi-hop routing between mobile MANET nodes in an ad-hoc network. The AODV protocol enables the mobile MANET nodes to obtain routes for new destinations quickly and does not require the MANET nodes to maintain routes to destinations that are not in active communication. The AODV protocol provides quick convergence when the ad hoc network topology changes (e.g., a new MANET node joins the network).
The AODV protocol uses a destination sequence number for each route entry. The destination node creates a destination sequence number for any usable route information the destination node sends to a requesting node. Using destination sequence numbers ensures loop freedom (i.e., prevents loops). Given a choice between two routes to a destination MANET node, a requesting node always selects one with the greatest sequence number. Advantageously, when the AODV protocol detects an RF link breakage, the AODV protocol immediately transmits notifications only to the affected set of nodes. Other nodes are not notified.
The current state of the art in MANET routing algorithms can be classified into two broad types: i) pro-active protocols and ii) reactive protocols. A proactive protocol typically uses a single aggregate value as the route cost between a source MANET node and a destination MANET node. Unfortunately, this means that fine-grained information is lost. Furthermore, the routing table in a pro-active protocol contains only one best route that is associated with the single aggregate value. This is a disadvantage because two different applications on the same node might have different quality-of-service (QoS) requirements. For example, an FTP application requires large bandwidth and a low error rate and tolerates high latency. However, a voice application requires low bandwidth and a low latency and tolerates a moderate number of errors. Using a single aggregate value to compute the best route cannot provide good service to both applications simultaneously.
A reactive protocol is a MANET routing algorithm that uses the Hop Count value as the metric for choosing the best route. However, research has shown that the Hop Count value alone is not a good routing metric. In addition, when a MANET routing algorithm is applied on a battery-operated device (e.g., cell phone), it is necessary to consider the effects of multi-hop routing on battery life.
Therefore, there is a need in the art for improved routing protocols for use in mobile ad hoc networks. In particular, there is a need for an improved routing protocol that does not rely on a single metric for determining the best routes from a source MANET node to a destination MANET node.